1. Field of the Invention
This invention relates to a toner used to develop an electrostatic latent image such as by electrophotography, electrostatic recording, electrostatic printing or similar techniques.
2. Description of the Prior Art
In the reproduction of copies from originals, generally an electrostatic image is first formed on a photosensitive medium or a dielectric film which is then developed by deposition with fine powder called a toner which is made of a colorant dispersed in a binder resin. After transferring the toner image to the surface of a support such as paper, the transferred image is perpetually fixed on the support surface such as by application of heat, to give a copy. A solvent treatment or a finish coating treatment has been used to fix a toner image, however, it is a more common practice to use a heat fixing method in which a toner image is heated up to a temperature at which the toner is capable of flowing so as to permit the toner to be fusingly attached to the support, thereby giving a final copy of the toner image.
In an attempt to speed up the duplication, a so-called heat roll fixing method has come into practice in which a toner image-bearing support is passed between a pair of rolls, i.e., a heat roll and a press roll, to fix it under heating and pressing conditions. The heat roll fixing method is higher in heat efficiency, lower in heat emission and quicker for fixation than other heat fixing methods, and is thus very effective. In this method, however, the toner image is brought to contact with the rolls, so that part of the toner image is apt to attach to the heat roll, causing the so-called offset phenomenon.
In order to inhibit the offset phenomenon, the surfaces of the rolls are usually covered with a material with an excellent releasing property such as a fluorine-contained resin and treated with an offset-inhibiting liquid such as a silicon oil. The application of the offset-inhibiting liquid in turn requires a device for consistently feeding a given amount of the offset-inhibiting liquid to the roll surface, coupled with the disadvantages that leakage of oil takes place and that silicon oil emanates an offensive odor on heating.
In recent years, attempts have accordingly been made to make the heat roll fixation feasible without use of any offset-inhibiting liquid. For instance, a method is described in Japanese Patent Publication No. 5549/78 (U.S. Pat. No. 3,666,247). Attempts to realize this feasibility by improvement of the toner are described in Japanese patent application (OPI) No. 65231/74 (British Pat. No. 1,442,835).
In Japanese Application (OPI) No. 65231/74, a mixture of a styrene resin and a low molecular weight polypropylene is used as a binder resin for the toner. On heating for fixation, the low molecular weight polypropylene turns into a liquid of low viscosity and is released on the toner surface where it covers the toner, and inhibits the offset phenomenon due to the releasing property of the polypropylene itself. According to this disclosure, it appears that offset-free, heat roll fixing is possible without the use of any offset-inhibiting liquid. However, this technique has a number of disadvantages. Poor compatibility of the styrene resin with the polypropylene results in the composition of toner particles becoming non-uniform (i.e., non-homogeneous). Furthermore, differences in the triboelectric charging properties of the toner particles leads to a reduction in developability, and results in a short lifetime or in a tendency for the toners to aggregate.
Accordingly, there is a need for a toner which is readily fixed and can prevent offset without the use of an offset-inhibiting liquid, and which exhibits good developability and cleanability and provides a homogeneous toner composition having good workability.